1. Field of the Invention
This invention relates to an electronic power switch in the form of a half bridge circuit having a series circuit of two power semiconductor groups between two direct, current connections, where at least one power semiconductor group consists of a parallel circuit of a plurality of individual power semiconductors of the same kind, where all power semiconductors of both groups are fastened on a common printed circuit board and where the power semiconductors are used in the switching operation.
2. Description of the Prior Art
An electronic power switch of this type is disclosed by German Patent DE 36 20 074 A1. Such electronic power switches, designed as half bridge circuits, are often employed in electrical technology. It is intended in the present case to limit the discussion purely to the switching operation and to greater outputs. This is meant to include a case where the current load carrying capacity of commercially available semiconductor components alone is not sufficient, so that a parallel switching arrangement of a plurality of semiconductor components of the same type is required, that is, for switchable outputs greater than 1 kW.
This situation occurs, for example, in welding machines, the output current of which may be on the order of several hundred Amperes. In dc welding machines, current control is often accomplished with a transistor chopper (that is, a half bridge circuit with a transistor group and a diode group) with the aid of a smoothing choke, customarily with frequencies beyond the audible range.
For this reasons, it is necessary to use very rapidly switching semiconductor components which must switch currents on the order of several hundred Amperes within a range of microseconds or even nanoseconds.
In the construction of such circuits, a mechanical layout having the minimum leakage inductance is most important because otherwise the reverse voltage load or the switching losses of the semiconductor components become too large. Large leakage inductances result from the fact that the individual semiconductor components are often attached to separate cooling plates. For this reason, it is necessary to provide additional damping elements, such as support condensers, which increase the structural size.
If, for ac welding machines, a complete transistor full bridge, that is, an inverter, with four controllable semiconductor components is required on the secondary side, using known construction, the total size will be so large that economical use becomes impossible to achieve. Therefore, this obvious principle of using a self-commutated secondary inverter based on known transistor technology has not been applied in connection with welding technology.
Thus, the problems associated with known prior art are the structural size required up to now, the high leakage inductance of the known construction and the very labor-intensive and complicated assembly requiring too many bus bars, cooling elements, condensers and attachment means resulting in too high a price.
As disclosed in German Patents DE 33 20 575 A1, DE 31 44 579 A1 and DE 85 08 595 U1, it is known to switch power semiconductor groups in parallel as well as to switch a plurality of power semiconductor groups in series. Diodes, transistors, thyristors or other power semiconductor components are usable as power semiconductors. Accordingly, it is possible to attach the drive circuits and possible supply and control circuits next to the power semiconductors on the same printed circuit board.
To provide a cooling bar with ribs is known from Germany Patent DE 38 02 593 A1, and German Patent DE 27 53 145 discloses a cooling bar cooled by means of a coolant.